1. Field of the Invention
The present invention relates to a semiconductor integrated circuit such as an LCD driver including a voltage-follower power circuit which supplies a given voltage to a load in a push-pull manner.
2. Description of the Related Art
Some power circuits for conventional LCD drivers employ a push-pull method as shown in FIG. 5. The power circuit shown in FIG. 5 includes a first amplifier circuit 100 which supplies a current from a power potential VDD of the high-voltage side to an output terminal by using a p-channel transistor of the output stage, and a second amplifier circuit 200 which allows a current to be absorbed into a power potential VSS of the low-voltage side from the output terminal by using an n-channel transistor of the output stage.
A first potential V10 and a second potential V20, which are obtained by dividing a given potential VL of the low-voltage side and a given potential VH of the high-voltage side by resistors R10, R20, and R30, are applied to the amplifier circuits 100 and 200, respectively. Since the lower first potential V10 is applied to the first amplifier circuit 100 and the higher second potential V20 is applied to the second amplifier circuit 200, the p-channel transistor of the output stage of the first amplifier circuit 100 and the n-channel transistor of the output stage of the second amplifier circuit 200 generally do not operate simultaneously.
However, when the power circuit is activated, the potential at each component is unstable. Consequently, the p-channel transistor of the output stage of the first amplifier circuit 100 and the n-channel transistor of the output stage of the second amplifier circuit 200 may operate simultaneously, which can undesirably cause a large current. Moreover, another problem is that an output voltage of the power circuit oscillates as if to ripple when increasing an offset of the first potential V10 and the second potential V20 by increasing the value of the resistor R20.
Japanese Unexamined Patent Application Publication No. 61-79312 describes a DC amplifier comprising an offset adjustment means which inputs DC elements included in the output of an amplifier to a window comparator, successively sends a control signal to a multiplexer by operating a comparison resistor upon exceeding a certain level, and controls the midpoint of the common source resistor of a first stage amplifier.
Japanese Unexamined Patent Application Publication No. 7-106875 describes a semiconductor integrated circuit comprising differential transistors, a transistor for current source connected to source electrodes connected to each other in the differential transistors, a resistor and a current source transistor for current source connected in parallel to the above-mentioned transistors, and a comparator which compares voltages of both ends of the resistor with a reference voltage and brings its output to the two transistors for current source.
However, the technologies described in these applications are not for controlling push-pull operations of the output stage, but for adjusting DC offsets of output potentials.
Taking the above-mentioned facts into consideration, an object of the present invention is to prevent a large current flowing from a p-channel transistor to an n-channel transistor of the output stage when a power circuit is activated in a semiconductor integrated circuit comprising the power circuit which supplies a given voltage to a load in a push-pull manner.